1. Field of the Invention
The present invention relates generally to turbidity suppression by optical phase conjugation using a spatial light modulator.
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
In general, biological tissues are highly turbid media in the optical regime [1]. The extensive scattering of light by tissue is a significant obstacle for deep-tissue optical imaging and optical sensing [2]. In recent years, several publications [3-6] have reported that it is experimentally possible to mitigate the effects of scattering by tailoring an input light field wavefront appropriately. For example, Mosk's group showed that it is possible to focus light through a scattering medium by modifying and optimizing the wavefront of an input light field with a spatial light modulator [3,5]. Our group showed that an optical phase conjugate (OPC) copy of an initial transmission through a biological sample can likewise undo the effects of the initial scattering [4,7,8].
Employing optical phase conjugation to suppress tissue turbidity as described in embodiments of the present invention is appealing because it simply requires the duplication of a transmission light field for which the phase at each point on the wavefront is sign-reversed. Optical phase conjugation (OPC) has been an active field since the 1970s and has produced numerous applications including novel resonators, high-resolution image projection, and optical computing devices [9-15]. The generation of the OPC wave was based on optical nonlinearities such as photorefractive effect [14] and Brillouin scattering [15]. OPC based on nonlinear light-matter interactions can handle a large number of optical degrees of freedom. However, the nonlinear optics based OPC techniques often provide limited phase conjugation reflectivity, defined as the power ratio of the phase conjugate signal to the input signal. In addition, specialized light source and nonlinear media are usually required [15]. For practical purposes, an OPC system that can work with various light sources of different wavelengths, coherence lengths, and power levels would be preferable.
Another promising class of optical methods is adaptive optics. In adaptive optics, a wavefront sensor and modulator are used to measure and compensate for phase aberration. Adaptive optics techniques were originally developed to compensate for atmospheric distortion in astronomical telescopes [16]. In the past two decades, several research teams have employed adaptive optics techniques to compensate for the aberration in the optical microscopy systems and the aberration induced by the refractive index variation in the specimen [17-19]. The amount of aberration in these applications is fairly limited and can often be decomposed to several orders of Zernike polynomials [17-20]. In such cases, deformable mirrors are often employed as the wavefront modulator to provide adequate aberration compensation. Recently Mosk's team has successfully demonstrated a pixel by pixel optimization method to form an optical focus through highly turbid samples (μsl ˜10, μs, scattering coefficient, l, sample thickness) by using high capacity spatial light modulators [3].
In view of the above, what is needed is the capability to mitigate the effects of scattering in a highly turbid media in a robust manner that can accommodate phase errors.